Traditions Training Cartridges are weighted and built like real cartridges. They include a rubber insert to function as snap caps.
Unlike most aluminum snap caps, however, they are heavy and strong enough to properly cycle, without becoming damaged.

I recently tested these in 12 gauge, .223 rifle and .45 ACP.

The .45 ACP pack contained 5 cartridges with 6 rubber inserts. They were "loaded" with 230 gr ball projectiles, and apart from their black coating, were almost indistinguishable from live ammo. I cycled these through an Auto Ordnance 1911 clone, through a dozen different magazines with no problems with the cartridges. In fact, they helped me identify two problematic magazines. They fed flawlessly. The hammer drop felt noticeably different due to the rubber primer insert, but there were no issues with the action. They ejected very positively, just like real cartridges.

I tested the pair of 12 gauge cartridges (two in the pack, inserts already in place) in both a side by side ERA coach gun and a Remington 870 riot gun. They held up well to the mechanical ejection, and loaded perfectly.

The two pack of .223 were tested in an M4 clone. Again, they cycled just like real ammo. I randomly loaded both into a magazine of live ammo to practice stoppage drills. There was no detectable difference in the load part of the cycle, and upon the hammer dropping, they extracted exactly as a dud round should.

These are a professionally made and tough test and training tool I recommend acquiring.

They are available in gauges/calibers of 10 semiauto pistol, 7 revolver, 5 shotgun and an incredible 42 rifle calibers, including most common hunting calibers and several military surplus calibers.

Manufacturer's suggested retail prices range from $9.98 for a dozen .22 long rifle caliber, to $15.98 for a single .50 BMG (which I very strongly recommend as part of your kit, given the power involved in these rifles).

These are so realistic, I also strongly recommend extra attention to safety. Do not keep them near your live ammo when performing function tests, and inspect carefully before loading. Always have the weapon pointed in a safe direction, and a backstop.

NOTE: I was furnished a pack each of .223, 12 gauge and .45 ACP free for evaluation.